Select Sales > Credit Memo to open the Credit Memo window.
Click the Lookup button next to the MemoNumber field to open the credit memo that needs to be voided.
Select the Invoices tab in Credit Memo window.
From the data grid list, select the credit memo invoice that needs to be voided.
Click the Void Invoice button (within the tab). A prompt will pop-up asking if you want to void the particular invoice. Click Yes to void it and proceed. Another prompt will pop-up confirming that the invoice has been voided.